The invention concerns a lens system, particularly for focusing electrons, with a cylindrical lens with pole shoes or electrodes, between which an aperture is located. This aperture has a slot-shaped cross section perpendicular to the optical axis of the lens.
With the aid of electron-beam lithography, structures of a minimal size can be created, a substantial field of application being the production of electronic components and integrated circuits on the surface of disk-shaped semiconductor crystals (wafers). To focus the electrons, electron lens are required that have a short focal distance and whose usable writing field (i.e. the range in which the focal point can be moved parallel to the lens axis) is as large as possible.
Known for this purpose are so-called xe2x80x98variable axisxe2x80x99 lenses (H. C. Pfeiffer and G. O. Langner, J. Vac. Sci. Technol., 1983, pages 3 to 14), where a suitable a correction field is dynamically superimposed onto a field of a circular lens which determines the position of the flying spot. However, it proves disadvantageous that the usable field of action of these lenses is limited by the bore radius of the pole shoes which is limited by the size of the focal distance. In practice, there are thus writing regions with diameters in the range of one millimetre. In order to be able to process integrated circuits of a standard size or even a whole wafer, conventional lithographic systems thus require, in addition to the electron optics, a table with workpiece clamping devices which can be moved in a plane perpendicular to the electron beam. In this case the efficiency is limited by the mechanical parameters of the table, where the minimum size of the creatable structures depends on the guide accuracy and the speed of movement of the table determines the maximum writing speed.
Furthermore, known in the state-of-the-art are both electrostatic as well as magnetic cylindrical lenses to focus the electrons (H. Rose, Optik 36, 1972, pages 19 to 36). Cylindrical lenses are provided with electrodes or pole shoes which serve to guide the electric or magnetic field, and between which there is a slot-shaped aperture through which extend the possible trajectories of charged particles in the lens. Here the longitudinal axis of the slot-shaped axis is aligned perpendicular to the optical axis of the lens; the plane formed by the optical axis and the longitudinal axis is termed hereinafter the centre plane. For stigmatic imaging, i.e. the punctual imaging of punctual electron emitters and parallel incident rays, cylindrical lens are however unsuitable, as they focus only perpendicular to the direction of the slot, while movable components of the particles parallel to the direction of the slot remain uninfluenced. Thus bar-shaped, astigmatic punctual patterns obtain which are insufficient for lithographic purposes.
Finally the movement of charged particles can also be influenced via quadripole fields. Perpendicular to its plane a magnetic quadripole also acts as a lens. Standard images, however, cannot be generated in this way because the influence of a quadripole on a planar, parallel beam cluster depends on its angular position to the magnetic field. While the quadripole lens focuses parallel beams in a plane, which comprises the optical angle, it acts in the plane perpendicular thereto, which comprises the optical axis, so as to defocus a parallel beam cluster. Solely the amount of the focus distances can be set equally in both planes. Owing to these characteristics, quadripole lenses are unsuitable for standard optical imaging.
Against this background, the object of the invention is to develop a lens system in which the apertures for the electron beam are of optional length in one dimension and which produces stigmatic imaging.
This task is solved therein that the cylindrical lens is combined with a magnetic lens with which a quadripole field can be produced, the lenses are arranged with little or no distance between them and their optical axes run parallel to one another, the quadripole lens has a slot-shaped aperture which is oriented parallel to the opening of the cylindrical lens, the focusing plane of the quadripole lens, which comprises the optical axis, is oriented parallel to the longitudinal axes of the apertures and its defocusing plane perpendicular to the longitudinal axes and the refractive power of the cylindrical lenses can be set at twice that of the quadripole lens.
The central idea of the invention consists therein to combine one cylindrical lens with one magnetic quadripole lens, where the term quadripole lens comprises all lens within the meaning of the invention by means of which fields of quadripole symmetry can be produced, for instance also eight-pole lens. The optical axes of both lens extend parallel to each other and their principal planes match or are at a small distance to each other. The term xe2x80x98smallxe2x80x99 is to be understood in the sense that the distance is small compared with the focusing length of both lens and the lens system can be described in good convergence by a single lens. Between its pole shoes the quadripole lens has a slot-shaped aperture, which is oriented parallel to the opening of the cylindrical lens, the length of both openings being optional. The projections of the two openings preferably match in the direction of the optical axis. The orientation of the quadripole lens is such that its focusing plans extends parallel to the longitudinal axis of the apertures and its defocusing plane perpendicular to the longitudinal axes. The refractive power of the cylindrical lens is suitably chosen so as to be twice that of the quadripole.
Resulting from this configuration, the quadripole lens provides a focusing effect in the plane of the slot where the focal distance is twice that of the cylindrical lens. Perpendicular to the direction of the slot the negative refractive power of the quadripole is overcompensated by the twice as high refractive power of the cylindrical lens. As a result a focal distance obtains which is also equal to twice that of the cylindrical lens. The result is thus a stigmatically focusing lens. Decisive for the coordination of the lens focal distances is solely that the sum of the refractive powers of the quadripole perpendicular to the slot longitudinal axis and the cylindrical lens is equal to the refractive power of the quadripole lens parallel to the slot longitudinal axis.
The advantage of the lens system consists therein that limitations of the beam path in the direction of the slot, as they exist in the case of circular lenses because of the bore radius of the pole shoe, are eliminated or substantially reduced. Nonetheless the system permits stigmatic imaging comparable to conventional circular lenses. In particular, no optical lens errors occur which are less than third order. Suitably in the majority of application cases the optical axis of the quadripole lens extends in the centre plane of the cylindrical lens. Furthermore it proves a significant advantage that along the slot, neglecting influences of the endwise slot edges, largely translation symmetry exists. Therefore the imaging characteristics are independent of the position of the optical axis of the quadripole in respect of the longitudinal axis of the slot. With suitable adjustment of the lens parameters, in particular the field intensities, imagings can of course also be produced with any other charged particles, for example ions which for instance are usable for the doping of semiconductors. Finally, it is conceivable for special applications that the coordination of the lens focal distances deviates from the above-mentioned setting, for instance to produce line-like structures on a workpeice with an astigmatic focal spot.
A preferred quadripole lens consists of one aperture on or at whose edges there extend current-carrying conductors. The conductors are oriented more or less parallel to the optical axis. Suitably the conductors here comprise coils wound on a yoke. The conductors or windings extend on the inside edges of the aperture, which face the optical axis, preferably without inclination, i.e. in the frame of production accuracy, exactly parallel to the optical axis in order to avoid distortions of the fields. On opposing edges of the aperture the direction of the currents is parallel, while the current flow on edges perpendicular thereto is oriented opposingly. Preferably the sum of all flows through the aperture is zero to avoid closed circulatory magnetic field lines, where the current densities on the short and long edges of slot-shaped aperture behave reversely proportional to the edge lengths. If the aperture is inserted in a material, for instance is enclosed by a yoke whose magnetic permeability is much higher than one, e.g. iron, the current intensity required to generate the fields is reduced.
As the simplest possibility for realising the necessary translation symmetry in the direction of the slot it is proposed that the apertures have square cross sections. As a result of the slot-shaped design the two edge lengths of the square differ considerably,
In an advantageous embodiment of the invention the optical axis of the quadripole lens can be moved in a parallel direction. Thus its position can be changed so that the parallel orientation to the centre plane of the cylindrical lens is always maintained. With a long length of the slot or suitable compensation of edge effects the imaging characteristics of the lens system are virtually uninfluenced when the position of the optical axis of the quadripole along the longitudinal axis of the slot changes. Conceivable, however, is also to move the optical axis of the quadripole lens perpendicular or at an optional angle to the slot longitudinal axis, that is, its distance to the centre plane of the cylindrical lens.
For the purpose of moving the optical axis of the quadripole lens it presents itself to superimpose a magnetic dipole field onto the quadripole field. Dipole fields parallel to the slot direction produce shifts of the optical axis perpendicular to the slot, while dipole fields perpendicular to the slot direction shift the optical axis in the direction of the slot longitudinal axis. When the dipole field is generated by electric current the continuous movement of the optical axis by any amount is possible by varying the strength of the field current. Suitably, two dipole fields that are orthogonal to each other and the optical axis can be superimposed onto the quadripole field, which allows optional movement across the entire surface of the slot cross section.
In order to produce the dipole field opposing edges of the aperture are preferably provided with conductors which can be impinged with currents which are opposing on both edges and of the same amount. Possibly for this purpose the aperture has special conductors or coils. Alternatively those conductors can be used which serve to produce the quadripolar field. In the latter case the current flows sum in individual conductors or compensate one another partly so that as a result the total current on each opposing edge deviates.
Furthermore, it is advantageous if higher order electric or magnetic multipole fields can be superimposed onto the quadripole field. For this purpose, the aperture edges of the quadripole lens are preferably provided with conductors or coils whose surface density varies along individual edge sections. By means of these generally additional conductors, with the corresponding arrangement, the quadripole field can be superimposed with any magnetic multipole fields of a higher order. They serve to correct spherical aberrations of the lens, the correction fields usually being considerably weaker than the dipole and quadripole fields.
To avoid a distance between cylindrical and quadripole lenses it is proposed to dispose the quadripole lens inside the cylindrical lens. Generally, one cylindrical lens consists of several pole shoe pairs or electrodes at a distance to one another. These parts can be disposed in front of and after the quadripole lens in the direction of the optical axis.
In a preferred embodiment of the invention, the pole shoes or electrodes of the cylindrical lens also consist of a material of high magnetic permeability. As a result they limit the magnetic field of the quadripole lens in the direction of the optical axis, with a two-sided limitation is effected provided that the quadripole lens is disposed inside the cylindrical lens. Of special advantage thereby are the plate-shaped pole shoes or electrodes of the cylindrical lens whose surfaces extend perpendicular to the optical axis. This avoids disturbing the symmetry of the quadripole field.
As cylindrical lenses, both electrostatic as well as magnetic lenses have proved themselves. An electrostatic lens consists of a number, for example three, successive electrodes in the direction of the optical axis with one each slot-shaped aperture. Thereby the electric potential of the successive apertures differs. To achieve a simple and compact lens system structure it is proposed that one of the electrodes comprises the surfaces of the pole shoes of the quadripole lens. With a cylindrical lens with three electrodes, the quadripole lens comprises preferably the central electrode. Suitably, not the entire pole shoe is impinged by the electrode potential rather its surface is provided with an insulated electrode opposite to it.
If in the case of an electrostatic lens the endwise electrodes have differing potentials, an immersion lens is obtained, which changes the kinetic energy of the particles passing through. This development then proves specially advantageous if low particle energy is planned in the image space.
A substantial expansion of the application range is obtained if the quadripole lens can be impinged by an electric field perpendicular to the optical. Thus additional dynamic effects can be applied to the electrons and in particular a Wien filter for electrons can be produced. Wien filters known in the state of the art consist of electrostatic and magnetic fields perpendicular to each other and effect a directional focusing, the location of the image depending on the kinetic energy of the charged particles. Accordingly, in conjunction with screens and deflection devices Wien filters serve to produce monochromatic electron beams. In the case of the lens system according to the invention, the electric field is oriented perpendicular to the longitudinal axis of the slot, the magnetic dipole field parallel to the longitudinal axis. The direction of the fields is chosen so that the forces are opposed to moved, charged particles, in the case of a defined particle speed both force fields being compensated. It shows itself to be advantageous that the resulting force in the slot is constant along the entire centre plane. The dispersion can be adjusted infinitely by changing the electric or magnetic fields.
The proposed lens system is especially preferred for purposes of electron or ion beam lithography. In this case it is oriented onto a workpiece so that its focus is located on its surface. A parallel beam generated by a charged particle emitter is thus focused onto the surface. The workpiece, in particular a wafer comprising semiconductor material, is affixed in a clamping device which can be shifted perpendicular to the slot direction and the optical axis. The advantage compared with the embodiments standard in the state of the art with circular lenses consists therein that a mechanical movement of the clamping device is necessary solely in one dimension. The use of a two-dimensionally movable clamping device is however not ruled out. The shifting of the focus on the surface of the workpiece in the direction of the slot can be realised through an axial shift of the quadripole lens, for instance through the superimposition of a dipole field. In a perpendicular direction thereto a mechanical displacement of the workpiece continues to be effected. However, with a grid-like guidance of the electron beam across the surface considerable advantages are obtained. This is explained therein that with a line-wise scanning the beam must be moved rapidly in a line direction, while a change of line is effected solely at the end of the line. The movement when the line is changed can be effected considerably more slowly owing the small path to be travelled. Thus the requirements on the movement speed and movement path of the workpiece clamping device are reduced particularly if the longitudinal axis of the slot extends in the direction of the lines of the grid. A considerable acceleration of the machining process as well as an improved accuracy of guidance are the advantageous consequences.